The microelectronics industry is constantly striving for further miniaturization of components to increase speed and functionality of electronic systems. This has led to the fabrication of highly complex integrated circuits on wafers of semiconductors such as silicon. In order to enhance the yield of usable integrated circuits on a given fabricated wafer, many processing techniques and apparatuses have been developed.
Wafer processing occurs in an enclosed vessel, often called a process chamber. Strict control of the process environment is required to keep the in-process wafer free of contaminants, and to expose it to necessary processing environments including specialized atmospheres for cleaning, etching, doping and deposition of materials through plasma processing such as sputtering or plasma etching.
One primary component of the wafer process chamber is the substrate support structure, often called a susceptor, or sometimes referred to as a “heater chuck” or simply “chuck”. The susceptor provides support for the wafer in the vessel and provides for sophisticated control of temperature during processing. Typically, the susceptor has a disk-shaped top plate upon which the wafer is held. The plate is supported by a central downwardly extending tube or pedestal. In some fabrication vessels, the plate can have an outer diameter of up to 55 centimeters.
It is well known that a susceptor can carry a number of structures including electrically resistive heating elements and thermal sensors such as “resistance temperature detector” (RTD) lines to help monitor and control the temperature of the wafer during processing. During wafer processing steps it is typically preferred to maintain the susceptor plate and the wafer at a high level of temperature uniformity. The heating elements and RTD sensors can be made up of a number of serpentine electrically resistive traces formed within the plate. The plate can be divided into a number of distinct zones, each having a dedicated number of separately energizable heating elements and sensors. Thus, a zone of a wafer located over a zone on the susceptor can be heated independently from the other zones. A hollow connecting tube or pedestal extends from the plate and is adapted to carry electrical power lines to the heating elements, electrical signal-carrying lines from the sensors, and electrical grounding lines for shielding structures. Other purposes of the pedestal can include pathways for process gasses, liquids, and vacuum channels into the chamber.
One problem with the above susceptor design is that the tube can only accommodate a limited number of wires to electrically interconnect large number of electrical elements on the plate due to geometric constraints.
Yushio et al., U.S. Patent publication No. 2003/0066587, incorporated herein by reference, teaches making a susceptor using aluminum nitride. Desai, U.S. Pat. No. 4,799,983, incorporated herein by reference, teaches the use of alumina in multilayer ceramic (MLC) technology.
In general, MLC technology involves mixing particles of high temperature-withstanding ceramic dielectric material such as alumina with an organic binder, which is then tape-cast, dried and separated into a number of flexible “green sheets”. Some of the sheets have via holes punched and are then screened and printed with metalization and other circuit patterns which, when stacked with other sheets, can form intricate three-dimensional electronic structures such as traces and contact pads. The stacked sheets are laminated together at a predetermined temperature and pressure to form a “green state” part. Depending on the type of ceramic particles used, the part can then be slowly heated in a binder burn-off routine to about 600 degrees C. which burns off a majority of the binder material. The resultant fragile baked-out or debound part is then fired at an elevated temperature routine in a reducing atmosphere such as humidified hydrogen-nitrogen upon which any residual amount of the binder material vaporizes off while the remaining material fuses and/or sinters into a solid ceramic body having embedded metallic electrical circuitry. Where alumina is used as the electrically insulating material, firing can typically reach 1600 degrees C. Refractory metals such as tungsten and molybdenum are often used for metalization.
Debound parts can contain significant void content corresponding to the removed binder. During the final firing process, the parts will shrink significantly as the material densifies. Shrinkage may not be uniform resulting in localized stresses on the part potentially leading to structural disuniformities including cracks. Two differently sized or shaped pre-fired ceramic parts would likely not be connected to one another and fired together because differences in the individual part shrinkages could cause breakage of the part-to-part joint.
Another problem faced by a designer of a susceptor is the corrosive gasses such as fluorine that are often used during etch processes within the vessel. This creates a harsh environment for the materials used in the vessel including most metals and metal oxides such as alumina. Thus special care must be taken in designing the internal structures of a susceptor apparatus to accommodate the presence of these destructive atmospheres.
One way to overcome the corrosiveness of these processing gases involves using Aluminum Nitride (hereinafter referred to as “AlN”) as the ceramic insulating material. AlN is resistant to corrosion by hot flourine gas and other reactive materials.
The coefficient of thermal expansion (“CTE”) or simply the thermal expansion of a material is defined as the ratio of the change in length per degree Centigrade to the length at 25 degrees C. It is usually given as an average value over a range of temperatures. The CTE of tungsten and AlN are similar at approximately 4.5 ppm/degree C.
Another consideration is that tungsten oxidizes readily at elevated temperatures when used in processes involving free oxygen. Thus it can be critical to isolate tungsten metallization from exposure to free oxygen during the sintering process and later use in high temperature applications. Thus, the tungsten should be hermetically sealed against such exposure. In typical susceptor applications the level of hermeticity should be adequate to prevent oxidation of tungsten during the expected operational life of the susceptor.
Another consideration involves the thermal conductivity of the materials in the susceptor. The thermal conductivity (“K” or “TC”) of a material is defined as the time rate of heat transfer through unit thickness, across unit area, for a unit difference in temperature or K=WL/AT where W=watts, L=thickness in meters, A=area in square meters, and T=temperature difference in degrees C. Thermal conductivity of a material is reduced with increase in temperature above 0 degrees. The pedestal is unheated and operates at a lower temperature compared with the plate, and as a result will have a higher effective thermal conductivity. Heat will be preferentially drawn away from the local region of attach and will upset the thermal uniformity of the heater plate.
The instant invention results from efforts to improve susceptor design using multilayer ceramic technology.